An electrodeless sensor for measuring the conductivity of aqueous solutions is known from the prior art in document DE 196 11 174 C1, where three transformers are used. A first transformer is connected to a liquid being measured and transforms a generator AC voltage into a magnetic field, and the liquid being measured is penetrated by the magnetic field which is produced by the first transformer from the generator AC voltage. A second and a third transformer are also provided, which are both coupled to the first transformer via the magnetic field that is present in the liquid. Each of the two additional transformers, which function as output transformers, is tuned to a different resonance frequency such that the two resonance frequencies are offset from each other. Addition of the two output voltages results in a wideband frequency response under the material influence of the liquid being measured. The output signals of the two additional transformers are analyzed to determine the conductivity of the liquid being measured, which is in the form of an aqueous solution, with any coupled interference signals being cancelled by appropriate circuitry.
Document DD 217 557 A1 discloses a method for controlling the addition of detergents or dish soap in washing machines, with various sensors being arranged in a washing machine in order to determine the physical and chemical properties of the detergent solution. Any change in the rise of the measurement signal during the addition of detergent or dish soap is detected and analyzed by an electronic circuit for analyzing the output signals of the sensors. The dosing of detergent for the washing machine can be controlled on that basis.
Document DE 197 55 418 A1, finally, discloses a sensor element and an apparatus for measuring complex impedances in materials, said sensor element comprising two electrodes made of a conductive material and arranged at a predetermined distance from each other. The two electrodes are covered with a thin insulating layer having a relatively small thickness in comparison to the predetermined gaps. The sensor element thus formed is largely insensitive to the surrounding media whose properties are to be detected. The output signals from the sensors are subjected to further processing in an evaluation circuit, and the properties of a respective liquid between the electrodes can be analyzed. More specifically, it is possible for complex impedances to be determined and analyzed as a measure of the liquid's properties.
Corresponding to the aforementioned known ways of detecting the properties of a detergent solution, for example, or of other fluid media, parameters for the properties of the media being studied can be specified with the devices described and by relatively complicated analysis of the measurement results. In particular, there is generally a strong need to detect material properties such as the quality of oil in an internal combustion engine, in order to change the oil when the properties of the oil being used deteriorates. A brake fluid in motor vehicles or a general hydraulic oil can likewise be checked for properties that can vary in operation.
The known ways, described in the foregoing, of detecting the properties of liquids such as a detergent solution or other aqueous solutions are devices and systems by means of which, in combination with a relatively complicated analysis of the measurement results and complex sensor units, the properties or at least individual parameters of the respective fluid being examined are determined.
Efforts to make a washing process both ecofriendly and optimized produce a strong need to detect the properties of a detergent solution in a washing device at the beginning of and throughout the washing process, so that the amount of water and/or the amount of detergent being used, for example, is precisely determined to create less pollution, and so that the amount of detergent for washing the laundry can be precisely dosed according to the level of contamination, or depending on particular properties of the water (mineral content, water hardness).
Systematic and correct dosing of detergent results in energy and water savings, as well as the desired reduction of wastewater pollution. A detection means for detecting detergent solution in a washing device, in respect of the detergent concentration or level of contamination, for example, should be of simple construction and operationally reliable. Since the aim is for more and more washing devices in private households and in industry to be equipped with a detection means for detecting the properties of the detergent solution, there is a strong need for sensor devices of simple construction that can be manufactured in large numbers at low cost and which can nevertheless provide reliable detection results for further analysis. Every washing device should ideally be equipped with such a detection means or sensor device, so that requirements for uncomplicated assembly during production of the washing device can also be taken into consideration, in addition to the requirement for simple construction.